Ohmic electrode structure, compound semiconductor light emitting device having the same and led lamp

ABSTRACT

An Ohmic electrode structure comprising a p-conductivity-type boron phosphide-based semiconductor layer containing boron and phosphorus as constitutional elements and having a surface; and an electrode disposed on said surface of said semiconductor layer and having an Ohmic contact with said semiconductor layer, wherein at least a surface portion of said electrode which is in contact with said semiconductor layer is formed from a lanthanide element or a lanthanide element-containing alloy. A compound semiconductor light-emitting device comprising a light-emitting layer formed of a compound semiconductor may advantageously comprise the Ohmic electrode structure

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofthe Provisional Application No. 60/458,950 filed on Apr. 1, 2003,pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a p-type Ohmic electrode structurewhich is to be provided on the surface of a p-conduction-type boronphosphide-based semiconductor layer so as to attain an Ohmic contactwith the layer, and to a compound semiconductor device employing thep-type Ohmic electrode structure. More particularly, the presentinvention relates to a technique for fabricating a compoundsemiconductor light-emitting device.

BACKGROUND ART

Conventionally, there have been disclosed techniques for fabricating aboron phosphide-based semiconductor device such as a light-emittingdiode (abbreviated as LED) from boron phosphide (chemical formula: BP),which is a type of Group III-V compound semiconductor, and a mixedcrystal thereof (see, for example, U.S. Pat. No. 6,069,021). Forexample, a p-conduction-type boron monophosphide (chemical formula: BP)layer is employed to serve as a barrier layer constituting alight-emitting portion having a pn-doublehetero (DH) junction structure(see, for example, Japanese Laid-Open Patent Application (kokai) No.2-288388). A boron phosphide-based semiconductor light-emitting diode iscomposed of, for example, a p-type cladding layer formed of a boronphosphide layer, and a p-type Ohmic electrode provided on the surface ofthe p-type cladding layer. In one conventional case, a p-type Ohmicelectrode provided on a p-type boron phosphide layer is formed fromaluminum (Al) (see, for example, K. Shohno et al., J. Crystal Growth,Vol. 24/25, 1974 (the Netherlands), p. 193).

Boron phosphide is known to provide either an n-conduction-type or ap-conduction-type low-resistance semiconductor layer, even when noimpurity is intentionally added thereto (see, for example, K. Shohno etal., J. Crystal Growth, Vol. 24/25, 1974 (The Netherlands), p. 193).Thus, an Ohmic electrode can be formed on a conductive boron phosphidelayer such as a cladding layer or a contact layer. In a conventionalcompound semiconductor light-emitting device having a magnesium (symbolof element: Mg)-doped p-type boron phosphide layer serving as a contactlayer, an Ohmic contact electrode is formed from gold (symbol ofelement: Au)-zinc (symbol of element: Zn) (as disclosed in, for example,Japanese Laid-Open Patent Application (kokai) No. 2-288388).

However, when the aforementioned metallic species are employed,formation of an Ohmic electrode exhibiting excellent Ohmic contactproperties with respect to p-type boron phosphide has not beensuccessfully achieved. Therefore, input resistance upon passage of anelectric current supplied for driving a light-emitting device (i.e.,device operation current) disadvantageously increases, resulting in anLED exhibiting high forward voltage (Vf), which is problematic. Suchhigh input resistance is also problematic for producing a laser diode(LD) having a low threshold voltage (Vth).

An object of the present invention is to provide a p-type Ohmicelectrode structure, to provide excellent Ohmic contact properties, of ap-type Ohmic electrode on the surface of a p-type boron phosphide-basedsemiconductor layer containing boron (B) and phosphorus (P) asconstitutional elements. The term “p-type Ohmic electrode” refers to apositive electrode which is provided on a p-type semiconductor layer.Another object of the present invention is to provide a compoundsemiconductor light-emitting device having a p-type Ohmic electrodehaving the electrode structure according to the present invention.

SUMMARY OF THE INVENTION

To attain the above objects, the present invention provides thefollowing.

(1) An Ohmic electrode structure comprising:

a p-conductivity-type boron phosphide-based semiconductor layercontaining boron and phosphorus as constitutional elements and having asurface; and

an electrode disposed on said surface of said semiconductor layer andhaving an Ohmic contact with said semiconductor layer, wherein at leasta surface portion of said electrode which is in contact with saidsemiconductor layer is formed from a lanthanide element or a lanthanideelement-containing alloy.

(2) The Ohmic electrode structure as described in (1) above, whereinsaid surface portion of said electrode in contact with said surface ofsaid semiconductor layer is formed from an alloy composed of lanthanumand an element having a work function of 4.5 eV or less.

(3) The Ohmic electrode structure as described in (1) or (2) above,wherein said surface portion of said electrode in contact with saidsurface of said semiconductor layer is formed from an alloy composed oflanthanum and aluminum.

(4) The Ohmic electrode structure as described in (1) or (2) above,wherein said surface portion of said electrode in contact with saidsurface of said semiconductor layer is formed from an alloy composed oflanthanum and silicon.

(5) The Ohmic electrode structure as described in any one of (1) to (4)above, wherein said electrode comprises a bottom layer of saidlanthanide element or lanthanide element-containing alloy which is incontact with said semiconductor layer, an intermediate layer of at leastone of titanium, molybdenum and platinum on said bottom layer, and a toplayer of gold or aluminum on said intermediate layer.

(6) A compound semiconductor device comprising said Ohmic electrodestructure as recited in any one of (1) to (5) above, wherein saidp-conductivity-type boron phosphide-based semiconductor layer is formedof p-type boron monophosphide which is undoped, where no impurity hasbeen intentionally added, and has a band gap between 2.8 eV and 5.4 eV,inclusive, at room temperature.

(7) The compound semiconductor light-emitting device comprising saidcompound semiconductor device as recited in (6) above.

(8) A compound semiconductor light-emitting device comprising:

a crystalline substrate formed of an insulating or conductive crystal;

a light-emitting layer formed of a compound semiconductor formed on saidcrystalline substrate;

a p-conductivity-type boron phosphide-based semiconductor layercontaining boron and phosphorus as constitutional elements and formed onsaid light-emitting layer, said p-conductivity-type boronphosphide-based semiconductor layer having a surface; and

a p-conductivity-type Ohmic electrode formed in contact with and havingan Ohmic contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer,

wherein at least a surface portion of said p-conductivity-type Ohmicelectrode which is in contact with said surface of saidp-conductivity-type boron phosphide-based semiconductor layer is formedfrom a lanthanide element or a lanthanide element-containing alloy.

(9) The compound semiconductor light-emitting device as described in (8)above, wherein said p-conductivity-type boron phosphide-basedsemiconductor layer is formed ofB_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ) (0≦α1, 0≦β<1, 0≦γ<1,0<α+β+γ≦1, 0≦δ<1) or B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1,0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1).

(10) The compound semiconductor light-emitting device as described in(8) above, wherein said p-conductivity-type boron phosphide-basedsemiconductor layer is formed of boron monophosphide (BP), boron galliumindium phosphide (compositional formula: B_(α)Ga_(γ)In_(1-α-γ)P: 0<α≦1,0≦γ<1) or a mixed-crystal compound containing a plurality of Group Velement in addition to boron and phosphorus.

(11) The compound semiconductor light-emitting device as described in(10) above, wherein said p-conductivity-type boron phosphide-basedsemiconductor layer is formed of boron nitride phosphide (compositionalformula: BP_(1-δ)N_(δ): 0≦δ<1) or boron arsenide phosphide(compositional formula: BP_(1-δ)As_(δ)).

(12) The compound semiconductor light-emitting device as described inany one of (8) to (11) above, wherein said surface portion of saidelectrode in contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer is formed from an alloy composed oflanthanum and an element having a work function of 4.5 eV or less.

(13) The compound semiconductor light-emitting device as described inany one of (8) to (12) above, wherein said surface portion of saidelectrode in contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer is formed from an alloy composed oflanthanum and aluminum.

(14) The compound semiconductor light-emitting device as described inany one of (8) to (12) above, wherein said surface portion of saidelectrode in contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer is formed from an alloy composed oflanthanum and silicon.

(15) The compound semiconductor light-emitting device as described inany one of (8) to (14) above, wherein said compound semiconductor layeris formed of a Group III-V compound semiconductor.

(16) The compound semiconductor light-emitting device as described inany one of (8) to (15) above, wherein said compound semiconductor layeris formed of gallium indium nitride (compositional formula:Ga_(x)In_(1-x)N: 0≦x≦1) or gallium nitride phosphide (compositionalformula: GaN_(1-y)P_(y): 0≦y≦1).

(17) The compound semiconductor light-emitting device as described inany one of (8) to (16) above, wherein said surface portion of saidelectrode in contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer is formed from a lanthanide elementor an alloy containing a lanthanide element, and has, as a planar shape,a pad electrode portion for establishing bonding and a net-like shapeportion extending from said pad electrode portion.

(18) The compound semiconductor light-emitting device as described inany one of (7) to (17) above, wherein said p-conductivity-type boronphosphide-based semiconductor layer is formed of p-type boronmonophosphide which is undoped, where no impurity has been intentionallyadded, and has a band gap between 2.8 eV and 5.4 eV, inclusive, at roomtemperature.

(19) An LED lamp employing said compound semiconductor device as recitedin any one of (8) to (18) above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing current-voltage characteristics of thematerial of the present invention and conventional materials.

FIG. 2 is a schematic cross-sectional view of the LED mentioned inExample 1.

FIG. 3 is a schematic cross-sectional view of the LED mentioned inExample 2.

FIG. 4 is a schematic plan view of the LED mentioned in Example 2.

MODE OF CARRYING OUT THE INVENTION

The present invention is directed to an electrode structure forproviding excellent Ohmic contact properties with respect to a p-typeboron phosphide-based semiconductor layer.

In the present invention, the term “boron phosphide-based semiconductor”refers to a compound semiconductor containing boron (symbol of element:B) and phosphorus (symbol of element: P) as constitutional elements.Specific examples include B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ)(0<α≦1, 0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1) andB_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1, 0≦β<1, 0≦γ<1,0<α+β+γ≦1, 0≦δ<1). Examples also include boron monophosphide (BP), borongallium indium phosphide (compositional formula: B_(α)Ga_(γ)In_(1-α-γ)P:0<α≦1, 0≦γ<1) and a mixed-crystal compound containing a plurality ofGroup V element species such as boron nitride phosphide (compositionalformula: BP_(1-δ)N_(δ): 0≦δ<1) or boron arsenide phosphide(compositional formula: BP_(1-δ)As_(δ)). The lower limit of thecompositional proportion of phosphorus (P); for example, (1−δ) in thecase of BP_(1-δ)N_(δ), BP_(1-δ)As_(δ), etc., is preferably 0.50 orhigher, more preferably 0.75 or higher.

The p-type boron phosphide-based semiconductor layer on which a p-typeOhmic electrode is provided can be formed through the halogen method,the hydride method, or MOCVD (metal-organic chemical vapor deposition).Alternatively, the semiconductor layer can be vapor-phase grown throughmolecular beam epitaxy (see J. Solid State Chem., 133 (1997), p.269-272). Specifically, a p-type boron monophosphide layer can be formedthrough MOCVD using triethylboran (molecular formula: (C₂H₅)₃B) andphosphine (molecular formula: PH₃) as sources. The p-type BP layer ispreferably formed at 1,000° C. to 1,200° C., and the source feed ratio(═PH₃/(C₂H₅)₃B) during layer formation is preferably controlled to 10 to50. The BP layer to which no impurity has been intentionally added;i.e., an undoped BP layer, effectively prevents deterioration of othercomponent layers caused by diffusion of impurities. By rigorouslycontrolling the formation speed as well as the formation temperature andthe V/III ratio, a boron phosphide-based semiconductor layer having awide band gap can be formed (see Japanese Patent Application No.2002-158282).

A p-type boron phosphide-based semiconductor layer having a band gap atroom temperature between 2.8 eV to 5.4 eV, inclusive, is particularlypreferably used. More preferably, a p-type boron phosphide-basedsemiconductor layer having a band gap as wide as 2.8 eV to 3.2 eV can beemployed as a barrier layer having a barrier effect; e.g., a p-typecladding layer, included in a compound semiconductor light-emittingdevice. Such a wide-band-gap p-type boron phosphide-based semiconductorlayer suitably forms a window layer which permits transmission, to theoutside of the light-emitting device, of visible light (blue, green,etc.) emitted from a light-emitting layer formed of gallium indiumnitride (compositional formula: Ga_(x)In_(1-x)N: 0≦x≦1) or galliumnitride phosphide (compositional formula: GaN_(1-y)P_(y): 0≦y≦1). Whenthe band gap is in excess of 5.4 eV, the barrier height with respect tothe light-emitting layer increases, which is disadvantageous inproduction of a compound semiconductor light-emitting device having lowforward voltage or threshold voltage. For example, the p-type claddinglayer is suitably formed from a low-resistance boron phosphide layerhaving a carrier concentration of 1×10¹⁹ cm⁻³ or more and a resistivityof 5×10⁻² Ω·cm or less, at room temperature. The thickness of the p-typeboron phosphide layer constituting the p-type cladding layer ispreferably controlled to 500 nm or more and 5,000 nm or less. In thestructure of a p-type Ohmic electrode to be formed so as to attaincontact with a p-type cladding layer, an excessively thin p-typecladding layer is unsuitable, because device operation current suppliedvia the Ohmic electrode cannot be uniformly diffused on the entiresurface of the light-emitting layer.

No particular limitation is imposed on the compound semiconductor devicewhich employs such a p-type boron phosphide-based semiconductor layer.However, a typical example is a boron phosphide-based semiconductor LED.Particularly, as mentioned above, the boron phosphide-basedsemiconductor layer is preferably used in combination with alight-emitting layer formed of gallium indium nitride (compositionalformula: Ga_(x)In_(1-x)N: 0≦x≦1) or gallium nitride phosphide(compositional formula: GaN_(1-y)P_(y): 0≦y≦1). In addition, the boronphosphide-based semiconductor layer can be used in a compoundsemiconductor light-emitting device such as a laser diode (LD).

In the present invention, the bottom surface of the p-type Ohmicelectrode to be in contact with the surface of the p-type boronphosphide-based semiconductor layer serving as a cladding layer, acontact layer for forming an electrode, or a similar layer is formedfrom a lanthanide element layer or from a layer of an alloy containing alanthanide element.

The term “lanthanide element” refers to a group of elements includinglanthanum (La, atomic number: 57) to lutetium (Lu, atomic number: 71)(see J. A. Duffy, “Inorganic Chemistry”, Hirokawa Shoten, published onApr. 15, 1971, 5th edition, p. 262). Elements such as cerium (Ce, atomicnumber: 58), praseodymium (Pr, atomic number: 59), neodymium (Nd, atomicnumber: 60), and holmium (Ho, atomic number: 67) are collectivelyreferred to as “lanthanoids” (see the above “Inorganic Chemistry,” p.263). Particularly, in the present invention, the bottom portion of thep-type Ohmic electrode is preferably formed from lanthanum (La) or alanthanum alloy, because, among lanthanoids, lanthanum and lanthanumalloys provide excellent Ohmic contact properties with respect to thep-type boron phosphide-based semiconductor layer. In addition, amonglanthanoids, lanthanum and lanthanum alloys provide most excellentjoining properties with respect to the boron phosphide-basedsemiconductor layer, whereby the bottom portion can be tightly joined tothe layer.

Alloys formed of lanthanum and a substance having a work function of 4.5eV or less are preferably used to form a bottom portion of the p-typeOhmic electrode which is to be in contact with the surface of the p-typeboron phosphide-based semiconductor layer. When the alloy has a workfunction in excess of 4.5 eV, the barrier height with respect to thep-type boron phosphide-based semiconductor layer steeply increases,which is disadvantageous in production of the p-type Ohmic electrode. Inaddition to having a large work function, an alloy of lanthanum and asubstance having a high melting point is advantageous for forming ap-type Ohmic electrode having excellent heat resistance. Therefore,alloys having high melting point are more preferable than lanthanumalloys with gallium (work function: 4.0 eV, melting point: 29.8° C.) orindium (work function: 3.8 eV, melting point: 156° C.), which have smallwork functions and low melting points. For example, the bottom portionexhibiting excellent Ohmic contact properties can be suitably formedfrom a lanthanum alloy with aluminum (work function: 4.3 eV, meltingpoint: 660° C.) or a lanthanum alloy with silicon (work function: 4.0eV, melting point: 1,414° C.). The aluminum (Al) or silicon (Si) contentis suitably 1% by mass or higher and less than 50% by mass. Suchlanthanum alloy film can be formed through use of means such as vacuumvapor deposition, electron beam vapor deposition, or high-frequencysputtering. The bottom portion having a desired planar shape such ascircular or square can be formed by patterning the alloy film through aconventional photolithography technique. Examples of such alloys furtherinclude lanthanum telluride (compositional formula: La₂Te₃), which is alanthanum alloy with tellurium (symbol of element: Te, work function:4.3 eV, melting point: 450° C.), and a lanthanum nickel alloy(compositional formula: LaNi₅), which is a lanthanum alloy with nickel(symbol of element: Ni, work function: 4.5 eV, melting point: 1,453°C.).

Characteristics of the p-type Ohmic electrode according to the presentinvention having a bottom portion formed of lanthanum or a lanthanumalloy can be investigated through, for example, a generally employedcurrent-voltage (I-V) characteristic profile. For example, FIG. 1 showsan I-V characteristic profile of a p-type Ohmic electrode formed from alanthanum-aluminum alloy (compositional formula: LaAl₂), as comparedwith those of a conventional electrode. The I-V characteristics weremeasured between p-type Ohmic electrodes arranged at an interval of 350μm. In each measurement, a p-type boron phosphide layer having aconstant electric resistance was used. Thus, a small resistancerepresents a small contact resistance. As shown in FIG. 1, under a givenapplied voltage, the LaAl₂ electrode according to the present inventioncan attain larger current flow, as compared with a conventional Ohmicelectrode formed of aluminum (Al), a gold (Au)-zinc (Zn) alloy, or agold (Au)-beryllium (Be) alloy. Thus, a p-type Ohmic electrode havingsmaller contact resistance can be produced.

In order to attain excellent contact between the Ohmic electrode and thep-type boron phosphide-based semiconductor layer, the p-type Ohmicelectrode; particularly, the bottom portion thereof, is preferablyformed from continuous film having no pores. Thus, the bottom portion ispreferably formed from lanthanum-containing film having a thickness of10 nm or more, more preferably 100 nm or more, most preferably 300 nm.

By stacking another metallic film on the upper surface of the bottomportion, an Ohmic electrode having a multilayer structure can befabricated. For example, in a preferred mode, titanium (Ti) film andgold (Au) film are sequentially stacked on the bottom surface (circularplanar shape) of an electrode member formed from lanthanum (La) (95 mass%)-aluminum (Al) (5 mass %) alloy film having a thickness of 120 nm,thereby forming a p-type Ohmic electrode of a triple-layer structure.When such a p-type Ohmic electrode having a stacked layer structure isfabricated, the uppermost layer is preferably formed from gold (Au) oraluminum (Al), so as to facilitate bonding. Furthermore, theintermediate layer included in a p-type Ohmic electrode of thetriple-layer structure and sandwiched by the bottom portion and theuppermost layer can be formed from platinum (Pt) or a transition metalsuch as titanium or molybdenum (Mo).

Through employment of the p-type Ohmic electrode of the structureaccording to the present invention, a compound semiconductorlight-emitting device having excellent electrical characteristics can beprovided. For example, an LED exhibiting small forward voltage (Vf) canbe provided. A visible-light-emitting, low-Vf LED can be formed by useof a stacked structure configuring sapphire substrate/n-type galliumnitride (GaN) cladding layer/n-type gallium indium nitride (GaInN)light-emitting layer/p-type boron phosphide (BP) cladding layer, whereina p-type Ohmic electrode formed of lanthanum-aluminum alloy film isprovided on the surface of the p-type BP cladding layer. In an LEDhaving a chip size of 300 μm to 350 μm, the bottom portion of the p-typeOhmic electrode preferably has a diameter of, for example, 90 μm to 150μm. When a Ga_(x)In_(1-x)N (0<x<1) layer of a multi-phase structureincluding a plurality of gallium indium nitride domains having indiumcompositional proportions (=1−x) that differ from one another isemployed, a compound semiconductor light-emitting device attaininghigher emission intensity can be effectively produced (Japanese PatentNo. 3090057).

An effective means for fabricating a large-area LED having a side lengthof, for example, 500 μm or more is to dispose the p-type Ohmic electrodeover a wide area of the surface of the p-type boron phosphide-basedsemiconductor layer. Such an Ohmic electrode is advantageous forproducing an LED with high emission intensity and wide emission area, asthe device operating current can be diffused over the wide area of thesurface of the light-emitting layer. The Ohmic electrode is preferablyformed in a shape such that the device operating current can be diffuseduniformly. For example, a p-type Ohmic electrode formed oflanthanum-aluminum alloy is disposed on the surface of the p-type borongallium phosphide mixed crystal layer provided on the light emittinglayer such that the p-type Ohmic electrode assumes a lattice shape or anetwork shape so as to establish electric contact. Alternatively, ap-type Ohmic electrode is fabricated from the bottom portion oflanthanum-silicon alloy provided so as to attain contact with the p-typeboron phosphide-based semiconductor layer and a portion extendingradially or in a branching-like manner from the bottom portion to theperiphery of the chip while electric conduction between the bottomportion and the electrode is maintained. Still alternatively, a p-typeOhmic electrode is fabricated from a plurality of concentriclanthanum-aluminum film members which are electrically connected withthe pad electrode disposed on the center of the p-type boronphosphide-based semiconductor layer. In the pad electrode for wirebonding which relates to any of the above p-type Ohmic electrodes, whenthe bottom portion of the pad electrode is formed from a material havinga high Ohmic contact resistance with respect to the p-type boronphosphide-based semiconductor layer, a flow of a device operatingcurrent to the bottom of the pad electrode from the directly underlyinglayer in a short circuit manner can be prevented, and advantageously, adevice operational current can be diffused over a wide area of thelight-emitting area opening to the outside so as to suitably permittransmission of emitted light.

In order to form a p-type Ohmic electrode having a desired planar shape,the entire surface of the p-type boron phosphide-based semiconductorlayer is covered with, for example, lanthanum-aluminum alloy filmthrough use of a conventional means such as vacuum vapor deposition orelectron-beam vapor deposition. Subsequently, the alloy film ispatterned into a desired shape through a conventional photolithographytechnique. The alloy film is preferably patterned into a shape such thatan equipotential distribution can be realized in the p-type boronphosphide-based semiconductor layer. Then, an unnecessary portion of thealloy film is removed through use of means such as wet etching or plasmadry etching using a halogen gas (e.g., chlorine (molecular formula:Cl₂)). Wet-etching of lanthanoid alloy can be performed by use of anacid mixture liquid containing glacial acetic acid (e.g., glacial aceticacid-hydrogen peroxide mixture liquid) (see, von Guenter Petzow(translated by Gentaro Matsumura), “Metal Etching Technique,” AGNE, Sep.10, 1977, 1st edition, 1st printing, p. 91).

An electrode having a bottom portion which is in contact with thesurface of the p-type boron phosphide-based semiconductor layer andwhich is formed of a lanthanide element or an alloy containing theelement can provide a p-type Ohmic electrode exhibiting excellent Ohmiccharacteristics with respect to the p-type boron phosphide-basedsemiconductor layer.

A p-type Ohmic electrode having a bottom portion which is in contactwith the surface of the p-type boron phosphide-based semiconductor layerand which is formed of a lanthanide element or an alloy containing theelement can diffuse device operation current over a wide portion of thelight-emitting area.

EXAMPLES Example 1

The present invention will be described in detail by taking as anexample a compound semiconductor LED fabricated by providing a p-typeOhmic electrode formed of a lanthanum-aluminum alloy (LaAl₂) on thesurface of a p-type boron phosphide-base semiconductor layer.

FIG. 2 schematically shows a cross-section of a stacked structureemployed for fabricating an LED 100 having a double-hetero (DH) junctionstructure. The stacked structure was fabricated by sequentiallydepositing, on a phosphorus (P)-doped n-type (111)-silicon (Si)single-crystal substrate 101, a lower cladding layer 102 formed ofundoped n-type boron phosphide (BP); a light-emitting layer 103 having amulti-layer quantum well structure including five units, each unitconsisting of a n-type gallium indium nitride (Ga_(0.90)In_(0.10)N) welllayer 103 a and a gallium nitride (GaN) barrier layer 103 b; and anupper cladding layer 104 formed of undoped p-type boron phosphide.

The undoped n-type and p-type boron phosphide layers 102 and 104 wereformed through atmospheric pressure (near atmospheric pressure)metal-organic vapor phase epitaxy (MOVPE) means by use of triethylboran(molecular formula: (C₂H₅)₃B) as a boron (B) source and phosphine(molecular formula: PH₃) as a phosphorus source. The n-type boronphosphide layer 102 and the p-type boron phosphide layer 104 were formedat 925° C. and 1,025° C., respectively. The light-emitting layer 103 wasformed through atmospheric pressure MOCVD means at 800° C. by use oftrimethylgallium (molecular formula: (CH₃)₃Ga)/NH₃/H₂ reaction system.The aforementioned gallium indium nitride layer, serving as the welllayer 103 a, was formed of a multi-phase structure comprising aplurality of gallium indium nitride phases which were different in theirindium compositional proportion. The average compositional proportion ofIn was found to be 0.10 (=10%). The thickness of the well layer 103 aand that of the barrier layer 103 b were controlled to 5 nm and 10 nm,respectively.

The carrier (hole) concentration and the thickness of the undoped p-typeboron phosphide layer 104 serving as the upper cladding layer 104 werecontrolled to be 2×10¹⁹ cm⁻³ and 720 nm, respectively. The layer 104 wasfound to have a resistivity of 5×10⁻² Ω·cm at room temperature. As thep-type boron phosphide layer 104 had a band gap of 3.2 eV at roomtemperature, the layer 104 was employed as a p-type upper cladding layeralso serving as a window layer through which light emitted from thelight-emitting layer 103 is transmitted to the outside.

On the entire surface of the p-type boron phosphide layer 104 serving asthe p-type upper cladding layer, a lanthanum-aluminum (LaAl₂) alloy film105, a titanium (Ti) film 106, and a gold (Au) film 107 were depositedthrough a conventional vacuum vapor deposition method and a conventionalelectron beam vapor deposition method. Subsequently, through employmentof a conventional photolithography technique, selective patterning wasperformed so as to leave the aforementioned triple-layer electrode witha bottom portion formed of the LaAl₂ alloy film 105, exclusively in anarea for providing a pad electrode 108 for bonding. Then, the LaAl₂alloy film and other film members present in the area except for thearea to be serve as the pad electrode 108 were removed through etchingby use of an acid-mixture liquid such as a glacial acetic acid-sulfuricacid-based liquid, whereby the surface of the p-type boron phosphidelayer 104 was exposed. After removal of the photoresist, selectivepatterning was performed so as to provide lattice-pattern grooves forcutting the structure into chips. Thereafter, and exclusively within thethus-patterned area, the p-type boron phosphide layer 104 wasselectively removed through the plasma dry etching method employing ahalogen mixture gas containing chlorine.

On the entire backside surface of the silicon single-crystal substrate101, an n-type Ohmic electrode (negative electrode) 109 was formed bydepositing a gold (Au) film by means of a conventional vacuum vapordeposition method. The structure was cleaved along the aforementionedstripe-like cutting grooves having a line width of 50 μm, which had beenprovided in a direction parallel to the <110> crystalline direction thatis normal to the (111)-crystal plane of the Si single-crystal substrate101, thereby providing square LED chips having a side length of 350 μm.

In the present invention, the bottom portion of the p-type Ohmicelectrode was formed from the lanthanum-aluminum (LaAl₂) alloy film 105having excellent bonding performance with respect to the p-type boronphosphide layer serving as the upper cladding layer 104. Therefore, thep-type Ohmic electrode 108 which was not peeled off from the p-typeboron phosphide layer 104 during wire bonding and which also served asthe pad electrode could be formed.

Emission characteristics of each LED chip 100 was confirmed upon passageof a device operation current of 20 mA in the forward direction betweenthe p-type Ohmic electrode 108 and the n-type Ohmic electrode 109. TheLED 100 emitted a blue light having an emission center wavelength of 440nm, with a half peak full width observed in the emission spectrum of 210meV. The luminous intensity of the LED chip, before being resin-moldedand as determined through a conventional photometric sphere, was 11 mcd.As the bottom portion of the p-type Ohmic electrode 105 was formed fromLaAl₂ alloy film having a small contact resistance with respect to thep-type boron phosphide layer, the forward voltage (Vf) at a forwardcurrent of 20 mA was found to be as low as 3.1 V and the reverse voltageat a reverse current of 10 μA was found to be as high as 9.5 V.Furthermore, an emission induced by a device operation current wasprovided from a virtually entire portion of the emission area except forthe projectional area of the pad electrode, because the upper claddinglayer was formed from a low-resistance undoped p-type boron phosphidelayer having a high carrier concentration, and a p-type Ohmic electrodeincluding a low-contact-resistance lanthanum-aluminum alloy film wasprovided so as to attain an Ohmic contact with the surface the p-typeboron phosphide.

Example 2

The present invention will be described in detail by taking as anexample a compound semiconductor LED fabricated by providing a p-typeOhmic electrode formed of a lanthanum-silicon alloy on the surface of ap-type boron gallium phosphide mixed crystal layer. The essentialstructure of the compound semiconductor light-emitting device of Example2 is the same as that of the LED of Example 1, and the same members arerepresented by the same reference numerals as employed in Example 1.FIG. 3 is a cross-section of and FIG. 4 is a plan view of the device ofExample 2.

On the light-emitting layer 103 mentioned in the aforementioned Example1, an undoped p-type boron gallium phosphide mixed crystal(B_(0.98)Ga_(0.02)P) layer 204 was deposited. The B_(0.98)Ga_(0.02)Player 204 was formed through reduced-pressure MOCVD means at 850° C.using a (C₂H₅)₃B/(CH₃)₃Ga/PH₃ system. The thickness of the layer wascontrolled to 340 nm. The carrier concentration and the resistivity ofthe B_(0.98)Ga_(0.02)P layer 204 were found to be 8×10¹⁸ cm⁻³ and 8×10⁻²Ω·cm at room temperature, respectively.

Subsequently, on the entire surface of the p-type B_(0.98)Ga_(0.02)Player 204, a lanthanum-silicon (La—Si) alloy film 205 was depositedthrough a conventional vacuum vapor deposition method. The thickness ofthe La—Si alloy film 205 was controlled to be 540 nm. Subsequently,patterning was performed through employment of a conventionalphotolithography technique and plasma etching technique. Then, anunnecessary portion of the alloy film was removed through the plasma dryetching method, whereby, as shown in FIG. 3, the La—Si alloy film waspatterned so as to provide a circle 205 (diameter: 150 μm) at the centerof the LED chip 200 and provide a network-like portion 210 around thecircle so as to attain contact with the surface of the p-typeB_(0.98)Ga_(0.02)P layer. On the thus-provided circle portion, anintermediate layer 206 and a gold film 207 were formed, thereby forminga pad electrode 208.

On the entire back surface of the silicon single-crystal substrate 101,a gold (Au) (99 mass %)-antimony (Sb) (1 mass %) alloy film 209 wasdeposited through a common vacuum vapor deposition method.

Subsequently, the thus-patterned La—Si alloy film and the deposited goldfilm were sintered at 450° C. for 10 minutes under a hydrogen flow whilethe films remained in contact with the stacked structure, therebyenhancing the Ohmic contact performance. Thus, the p-type Ohmicelectrode 205 formed of La—Si was provided on the surface of the p-typeB_(0.98)Ga_(0.02)P layer 204, and the Au Ohmic electrode 209 was formedon the backside of the silicon single-crystal substrate 101. The Au filmserving as the n-type Ohmic electrode 209 was controlled to 2 μm.

Subsequently, cutting was performed along the cutting lines which hadbeen provided during etching of the aforementioned La—Si alloy film, toproduce individual LED chips. The cutting grooves were provided inparallel to the crystalline orientations of the Si single-crystalsubstrate 101 of [1.−1.0] and (−1.−1.0), whereby individual LED chips200 in the form of rectangular having side lengths of 500 μm and 600 μmwere produced. When a forward current of 20 mA was caused to flowbetween the p-type and the n-type Ohmic electrodes 205 and 209 of eachlarge-scale LED chip 200, the emitted light had an emission centerwavelength of 440 nm. A near-field light emission pattern indicated thatthe emission intensity was uniform on the entire portion of the emissionarea except for the area of the pad electrode disposed at the center ofthe chip 200. Such emission was attained because the p-type Ohmicelectrode was provided in such a manner that the device operationcurrent can be uniformly diffused in a wide area of the p-typeB_(0.98)Ga_(0.02)P layer. The forward voltage at a forward current of 20mA was found to be 3.4 V and the reverse voltage at a reverse current of10 μA was found to be 8.3 V.

INDUSTRIAL APPLICABILITY

According to the present invention, a p-type Ohmic electrode which isprovided so as to attain an Ohmic contact with the surface of a p-typeboron phosphide-based semiconductor layer is formed from a lanthanideelement or an alloy film containing a lanthanide element, and anelectrode having a low contact resistance can be formed, resulting ineffective utilization of a device operation current supplied for lightemission. Therefore, a compound semiconductor light-emitting deviceattaining a high emission intensity can be produced. Furthermore, byconnecting a lead wire to the compound semiconductor light-emittingdevice according to the present invention, and molding the assembly witha resin, a high-luminance LED lamp can be produced.

1. An Ohmic electrode structure comprising: a p-conductivity-type boronphosphide-based semiconductor layer containing boron and phosphorus asconstitutional elements and having a surface; and an electrode disposedon said surface of said semiconductor layer and having an Ohmic contactwith said semiconductor layer, wherein at least a surface portion ofsaid electrode which is in contact with said semiconductor layer isformed from a lanthanide element or a lanthanide element-containingalloy.
 2. The Ohmic electrode structure as set forth in claim 1, whereinsaid surface portion of said electrode in contact with said surface ofsaid semiconductor layer is formed from an alloy composed of lanthanumand an element having a work function of 4.5 eV or less.
 3. The Ohmicelectrode structure as set forth in claim 1, wherein said surfaceportion of said electrode in contact with said surface of saidsemiconductor layer is formed from an alloy composed of lanthanum andaluminum.
 4. The Ohmic electrode structure as set forth in claim 1,wherein said surface portion of said electrode in contact with saidsurface of said semiconductor layer is formed from an alloy composed oflanthanum and silicon.
 5. The Ohmic electrode structure as set forth inclaim 1, wherein said electrode comprises a bottom layer of saidlanthanide element or lanthanide element-containing alloy which is incontact with said semiconductor layer, an intermediate layer of at leastone of titanium, molybdenum and platinum on said bottom layer, and a toplayer of gold or aluminum on said intermediate layer.
 6. A compoundsemiconductor device comprising said Ohmic electrode structure asrecited in claim 1, wherein said p-conductivity-type boronphosphide-based semiconductor layer is formed of p-type boronmonophosphide which is undoped, where no impurity has been intentionallyadded, and has a band gap between 2.8 eV and 5.4 eV, inclusive, at roomtemperature.
 7. The compound semiconductor light-emitting devicecomprising said compound semiconductor device as recited in claim
 6. 8.A compound semiconductor light-emitting device comprising: a crystallinesubstrate formed of an insulating or conductive crystal; alight-emitting layer formed of a compound semiconductor formed on saidcrystalline substrate; a p-conductivity-type boron phosphide-basedsemiconductor layer containing boron and phosphorus as constitutionalelements and formed on said light-emitting layer, saidp-conductivity-type boron phosphide-based semiconductor layer having asurface; and a p-conductivity-type Ohmic electrode formed in contactwith, and having an Ohmic contact with, said surface of saidp-conductivity-type boron phosphide-based semiconductor layer, whereinat least a surface portion of said p-conductivity-type Ohmic electrodewhich is in contact with said surface of said p-conductivity-type boronphosphide-based semiconductor layer is formed from a lanthanide elementor a lanthanide element-containing alloy.
 9. The compound semiconductorlight-emitting device as set forth in claim 8, wherein saidp-conductivity-type boron phosphide-based semiconductor layer is formedof B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)As_(δ) (0<α≦1, 0≦β<1, 0≦γ<1,0<α+β+γ≦1, 0≦δ<1) or B_(α)Al_(β)Ga_(γ)In_(1-α-β-γ)P_(1-δ)N_(δ) (0<α≦1,0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ<1).
 10. The compound semiconductorlight-emitting device as set forth in claim 8, wherein saidp-conductivity-type boron phosphide-based semiconductor layer is formedof boron monophosphide (BP), boron gallium indium phosphide(compositional formula: B_(α)Ga_(γ)In_(1-α-γ)P: 0<α≦1, 0≦γ<1) or amixed-crystal compound containing a plurality of Group V element inaddition to boron and phosphorus.
 11. The compound semiconductorlight-emitting device as set forth in claim 10, wherein saidp-conductivity-type boron phosphide-based semiconductor layer is formedof boron nitride phosphide (compositional formula: BP_(1-δ)N_(δ): 0≦δ<1)or boron arsenide phosphide (compositional formula: BP_(1-δ)As_(δ)). 12.The compound semiconductor light-emitting device as set forth in claims8, wherein said surface portion of said electrode in contact with saidsurface of said p-conductivity-type boron phosphide-based semiconductorlayer is formed from an alloy composed of lanthanum and an elementhaving a work function of 4.5 eV or less.
 13. The compound semiconductorlight-emitting device as set forth in claim 8, wherein said surfaceportion of said electrode in contact with said surface of saidp-conductivity-type boron phosphide-based semiconductor layer is formedfrom an alloy composed of lanthanum and aluminum.
 14. The compoundsemiconductor light-emitting device as set forth in claim 8, whereinsaid surface portion of said electrode in contact with said surface ofsaid p-conductivity-type boron phosphide-based semiconductor layer isformed from an alloy composed of lanthanum and silicon.
 15. The compoundsemiconductor light-emitting device as set forth in claim 8, whereinsaid compound semiconductor layer is formed of a Group III-V compoundsemiconductor.
 16. The compound semiconductor light-emitting device asset forth in claim 8, wherein said compound semiconductor layer isformed of gallium indium nitride (compositional formula:Ga_(x)In_(1-x)N: 0≦x≦1) or gallium nitride phosphide (compositionalformula: GaN_(1-y)P_(y): 0≦y≦1).
 17. The compound semiconductorlight-emitting device as set forth in claim 8, wherein said surfaceportion of said electrode in contact with said surface of saidp-conductivity-type boron phosphide-based semiconductor layer is formedfrom a lanthanide element or an alloy containing a lanthanide element,and has, in a planar shape, a pad electrode portion for establishingbonding and a net-like portion extending from said pad electrodeportion.
 18. The compound semiconductor light-emitting device as setforth in claim 7, wherein said p-conductivity-type boron phosphide-basedsemiconductor layer is formed of p-type boron monophosphide which isundoped, where no impurity has been intentionally added, and has a bandgap between 2.8 eV and 5.4 eV, inclusive, at room temperature.
 19. AnLED lamp employing said compound semiconductor device as recited inclaim 8.